This study examines the effects of somatropin therapy on bone mineral density in patients with adult-onset growth deficiency. The following hypotheses will be tested: Patients with adult-onset growth hormone deficiency who are treated with somatropin will have a greater bone mineral density (BMD) of the spine compared to patients who are treated with placebo, at the end of 2 years of treatment. Patients with adult-onset growth hormone deficiency who are treated with somatropin will have a greater bone mineral density in the hip than patients who are treated with placebo, at the end of 2 years of treatment. Patients with adult-onset growth hormone deficiency who are treated with somatropin will demonstrate increased bone formation and decreased bone breakdown, as assessed by serum and urinary markers, compared to patients treated with placebo. The specific aims are to: Compare effects of somatropin therapy vs. placebo therapy on bone mineral density and serum and urinary markers of bone formation and bone breakdown in a randomized trial of patients with adult-onset growth hormone deficiency over two years of treatment; Assess adverse events associated with 2 years of somatropin treatment in patients with adult-onset growth hormone deficiency; Correlate BMD changes with changes in total body composition; and, Collect data on a new health-related quality of life questionnaire for somatropin deficiency syndrome.